Grid: disable auto subdivision
Subdivisions option in the Display rollout controls how the grid subdivides while zooming in the preset orthographic view:


  Subdivisions
   Controls the number of sub-lines that appear in each cell of the grid. In aligned orthographic views the level of subdivision depends on the zoom.


To stop the grid from subdividing at all set amount of subdivisions to 1. You might want to adjust Scale option to make the grid less big:



Note that this won't work in any other unit system except for the None which is by default, because both Metric and Imperial already have predefined spacing between grid cells so they aren't adjustable.